Apple provides a standard warranty of one year on all the products they sell. You may purchase AppleCare any time within that one year period to extend your coverage, which for Macs is two additional years, for a total of three. The total coverage period begins from the date of purchase of your MacBook.
Waiting to buy it means you have money in your pocket, but go without phone support once your 90 day complimentary support expires. Your coverage is pinned to the date of purchase of the Mac - not of the plan. Also, Apple offers a pro-rated refund if you wish to cancel the plan before it's term is due. This is handy when you sell and the buyer doesn't want that protection or something like a loss or uncovered accident means you don't want to repair the out of warranty issues to benefit from the plan going forward.
This is the route I tend to do, as I never need phone support, and the cost of AppleCare is usually around $200 more. Of course, you run the risk of forgetting and not getting it, as I have done on more than one occasion.